Dissociation of an acid

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Homework Statement:

Calculate the pH for 1M HCN dissolved in water, and for 0.1M HCN dissolved in water. Is the acid most dissociated at high or at low pH?

Relevant Equations:

.
So I calculated the pH for the two concentrations: pH=4.61 for the high concentration and pH=5.61 for the low concentration.

Regarding the dissociation: in the solution manual it is given that the acid is most dissociated at the higher pH than the lower pH, which to me seems strange intuitively! any good explanation of this?
 

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  • #2
Ygggdrasil
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Write out the equation for the dissociation of HCN then consider Le Chatelier's principle.
 
  • #3
symbolipoint
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Second question seems a bit unclear. Simple response, pH less than 7 if HCN is found dissolved in water, and no other salts added.

HCN + H2O⇔H3O+ + CN-

Ka is 4.9*10^-10 .

For the 1 M HCN part of the problem,
x, H+1 molarity(approximate) as well as CN-1 molarity
1-x, HCN molarity


(x^2)/(1-x)=4.9*10-10


Try to work with that.

---
EDIT: small edit for subscript
 
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  • #4
Borek
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I suppose what the statement refers to is the fact that ratio [itex]\frac{[A^-]}{[HA]}[/itex] goes up with pH.
 
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Yeah, that's correct @Borek. So we basically get more of the products at higher pH? I just find it weird how the acid is most dissociated at a HIGHER pH, when the concentration of [H+] will be higher at the lowest pH?
 
  • #6
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High concentration of H+ doesn't have to come from the dissociation of the acid in question, imagine putting HCN in a solution of HCl - low pH, and HCN dissociation shifted way to the left (even more than usually, it is a very weak acid).
 
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Is it always like that? The higher the pH, the more of an acid will be dissolved? How can this be seen in light of Le Chatelier's principle? (as pointed out by @Yggdrasil)
 
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Anyone..?
 
  • #9
Ygggdrasil
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Is it always like that? The higher the pH, the more of an acid will be dissolved? How can this be seen in light of Le Chatelier's principle? (as pointed out by @Yggdrasil)
Try writing out the chemical equation for the dissociation of a weak acid and applying Le Chatelier's principle to see what happens as you increase or decrease the pH.
 
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Ok, so if we have the following reaction:

HA <--> H+ + A-

Then if we decrease the pH - concentration of [H+] will increase, reaction will be shifted towards the left => less products.

If we increase the pH - concentration of [H+] will decrease, reaction will be shifted towards the right => less reactants.

Is this the correct way of explaining it?
 
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  • #11
symbolipoint
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Yes, or that seems to be the correct use of the concept.
 
  • #12
epenguin
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Is it always like that? The higher the pH, the more of an acid will be dissolved? How can this be seen in light of Le Chatelier's principle? (as pointed out by @Yggdrasil)
Anyone..?
You ask for answers but you also need to give some. E.g. the answers to #3 and what are your results you claim in #1 and how did you get them?

In your question you seem to be confusing "disassociation" with "dissolving". Two different things, each with a clear meaning (but you wouldn't be the first).

Confusingly maybe there is for, as it were secondary reasons a connection. Many acids are less soluble than the corresponding salts. Because ionised i.e. charged species interact with water better than the corresponding uncharged one. More than once I have seen a beginner or a technician think they are going to dissolve some organic or other acid and then bring it to pH to make a buffer. They tip the acid ( powder or crystals) into a beaker with water and leave it stirring. After an hour or a day they say " it won't dissolve!". They need to start adding alkali for that to happen and to get a buffer of the concentration required!
 
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